The present invention relates to a resistance heating element for use in an electric furnace. More particularly, the present invention relates to a continuous resistance heating element of the ribbon type that can be bent into various shapes to fit different electric furnaces and when bent substantially retains the same cross-sectional configuration along its entire length.
Conventional electric furnaces generally have one of the following resistance elements: a series of groups of parallel spaced wire sections supported within ceramic blocks or similar supporting means or a single continuous element folded into a series of spaced sections as illustrated for example in U.S. Pat. Nos. 4,147,888 and 1,928,142.
The first type of element, the spaced wire sections, requires numerous thick supports which generally are grooved with the wire being supported in the grooves. The use of this type of support has disadvantages in that it is inefficient because the support absorbs the heat created by the element until the support is the same temperature as the element. Further, the wire sections and supports add additional expense to the manufacturer of the furnace since each wire section and support must be individually made to appropriate lengths for each specific furnace.
The second type of element, the continuous resistance element, generally reduces the loss of heat through absorption because the element can be supported at its ends where the folds are made. This allows a larger area of the resistance element to be exposed making the element more efficient.
A problem with the continuous resistance element is the way the element is folded to conform to the furnace. At each fold, the element overlaps itself which increases the element density at that point and the resistance. Increased resistance increases the wattage at the fold which increases wattage per density causing the element to burn out at the folds reducing the life of the element.
In Beck, U.S. Pat. No. 4,011,395, an electric furnace heater is illustrated having a flat resistor ribbon which is bent into a serpentine pattern and does not have the problem of overlap at the bends. However, in order to support the ribbon the heater is formed with transverse rows, each row having a plurality of spacers and brackets to support the ribbon. The brackets are welded to the ribbon which increases the density of the ribbon at that point which can lead to a reduction in the life of the ribbon. Also, the necessity of welding each bracket is time consuming and makes replacement of the ribbon difficult.